Training nutrition for runners: Part 1 | Part 2 | Part 3 | Part 4 | Part 5 | Part 6
How Low-Carb Diets Affect Endurance Performance in Runners
- Sports nutrition
- Low-carb
- High-carb
Thomas Solomon, PhD.
This page contains an evidence-based explanation of whether a low-carb, high-fat ketogenic diet improves endurance performance, using the Supernova study series in elite race walkers as the main example.
The content is for runners and endurance athletes who want a clear, research-based answer on low-carb diets, race performance, running economy, and fuel use during hard efforts.
Key takeaway: In the Supernova trial, a low-carb ketogenic diet increased fat burning during exercise, but it also reduced exercise economy, so the athletes needed more oxygen to hold the same speed.
Key takeaway: The high-carb diet improved the exercise economy and 10 km race performance, while the low-carb diet did not improve race performance.
Key takeaway: Follow-up Supernova studies found the same pattern again, and the slower performance after short-term low-carb adaptation was not fully restored by bringing carbohydrates back for one day or even for two weeks, which is kinda a big deal for race-focused athletes.
But don't stop there! Scroll down to fully educate yourself on the details, nuances, and nerdy bits.
Last time, I took you on a journey through the 100-year research history that has shaped our knowledge concerning the effects of low-carb/high-fat diets on exercise performance. But I didn’t conclude the story; I left you with an important question: “does a low-carb/high-fat diet offer a performance advantage to an endurance athlete?”. Stay with me as I begin to answer this complex question, which Liam Gallagher started singing about in the 90s...
Let’s dive in…
You might be surprised to find that it is among some immortal words echoing the Britpop scene of the late 90s where we will find some clarity in this never-ending story of the low-carb diet puzzle.
“Slowly walking down the hall. Faster than a cannonball.”
Back in 1996, I literally had no idea what Liam Gallagher was whining about. But now, I am sure he must have written those lyrics while coming off the back of a low-carb/high-fat diet bender…
Why do I say that?
Well, to bring clarity to this fatty mess, all it took was the best low-carbohydrate diet and exercise performance study to date.
Champagne Supernova.
What on Earth am I talking about?
Simple.
Louise Burke’s masterpiece: the Supernova clinical trial…
The last 100 years of research teach us that habituating to a low-carbohydrate diet decreases muscle glycogen levels while increasing fat oxidation rates during exercise and prolonging running and cycling time-to-exhaustion at low exercise intensities. But, knowing your “time to exhaustion” is not ideal since it does not simulate race performance. And endurance races are generally not run at a low intensity — you never line up to “give it medium”.
So, I concluded my last post by stating that to test the hypothesis that a low-carb/high-fat diet offers a performance advantage to an endurance athlete, we would need a well-controlled trial examining the effects of habituation to a low-carb vs. a high-carb diet on race performance in a large group of highly trained endurance athletes. In 2016, Louise Burke and her team did just that — they invited world-class endurance athletes to join "inpatient" training camps at the Australian Institute of Sport for 3-weeks while intensively training and receiving either a low-carbohydrate ketogenic diet (less than 50 g/day, 2.1 g/kg/day of protein; identical to that used by Phinney et al. in 1983) or an energy-matched high-carbohydrate diet (8.6 g/kg/day of carbohydrate, 2.1 g/kg/day of protein). Before and after the 3-week interventions, athletes not only completed a 25 km bout to assess fat and carb oxidation rates and economy but also competed in an official World Athletics-sanctioned 10 km race.
Burke et al. (2016) J Physiol.
What did they find?
First off, the 3-week intensive training period increased maximal aerobic power no matter which diet had been consumed. Adaptation to the ketogenic low-carbohydrate, high fat diet increased fat oxidation rates during exercise at a range of exercise intensities. However, the low-carb diet-induced increase in fat oxidation was coupled with reduced economy, i.e. increased oxygen demand at a given speed. Meanwhile, the high-carb diet decreased the oxygen costs of exercise and improved 10 km race performance by 6.6%, while race performance was not improved following habituation to the low-carb diet.
To summarise, the Supernova study showed that world-class endurance athletes do not race faster when habituated to a low-carb ketogenic diet and become less economical — they use more oxygen to move at the same speed.
So, as it turns out, world-class endurance athletes who are highly-trained and, by extension, highly “fat-adapted”, still need high carbohydrate availability to perform at their best. The data are so clear that Burke and her colleagues gave the paper a very self-explanatory title, “Low carbohydrate, high fat diet impairs exercise economy and negates the performance benefit from intensified training in elite race walkers”.
Was that the real “nail in the coffin”?
Of course it wasn’t.
As expected, the paper went viral and, in some cases, prompted mayhem. Among some of the kinder critiques, a couple of reasonable but somewhat nitpicky points were raised, all of which were well-addressed by a rebuttal from Burke’s team. Unfortunately, there are also folks who “take a side” in the low-carb camp and use Darth Vader levels of hatred on Twitter to try to bring down Burke and her team’s valiant effort.
Contrary to some popular beliefs, good scientists like Burke are not out on an agenda-driven quest to sink keto as a dietary approach to improve performance. Some scientists are also practitioners, looking to optimize their athletes’ performance. In Burke’s case, she is a practitioner at the AIS trying to optimize the performance of Australian Olympians (or, at least she was until she left her role at the AIS a few months ago).
The bonkers comments can also get somewhat amusing at times. For example, “Phinney clearly showed that low-carb works” or, “Burke’s studies simply aren't long enough to cause fat-adaptation.”
Why is this amusing?
Well, because humans “fat adapt” rather rapidly to low-carb/high-fat feeding — we’ve known for 100-years that resting RER values (respiratory exchange ratios) drop within days of beginning a low-carb/high-fat diet, indicative of greater fat oxidation rates at rest. Furthermore, it seems that in the eyes of the “Burke haters”, Phinney’s and Volek’s studies and the suggested approaches outlined in their popular science book, The Art and Science of Low Carbohydrate Performance are long-enough to cause fat adaptation, but when Burke’s team use the exact same diet for the exact same time, their studies are too short to cause fat adaptation.
As I said, bonkers.
Yes, there were some limitations, such as the lack of randomisation, the permission for participants to choose their group allocation, and that race walking might be hard to relate to for athletes in other endurance sports. But, as Burke and her team were very up-front about, they did their best to create “a pragmatic blend of rigorous scientific control and research methodology with real‐world allowances needed to accommodate elite athlete populations”.
Potentially ruining Olympic medal hopes of a world-class athlete by randomising them to something potentially detrimental is not ideal. So, the Supernova study purposely recruited athletes who were genuinely interested in the dietary approaches being offered and allowed them to choose their dietary group allocation, providing the potential for their “belief” in the benefits of the intervention to play a role — this is the “placebo effect” and is something many athletes try to use to their advantage.
Neither a scientist, a practitioner, or an athlete, could not deny that Supernova is a well-designed and well-conducted study. It has limitations, yes. But, when a good scientist agrees there are some limitations with their work, and when their work’s conclusions have many nay-sayers, and when that same scientist has received a lot of hate, what would that scientist do?
They would nod their head, smile, and do it all again... Supernova 2.
As Alex Hutchinson so eloquently put it: “The most convincing way to refute Burke’s results isn’t to list all the things she might have done wrong; it’s to do it right, demonstrate the performance benefits, and publish the results. Better yet, do it twice.”
And that she did.
To appease the nay-sayers, Burke and her team took heed of the limitations (even publishing their own appraisal) and did it all again. Welcome to Supernova 2...
Open access licensed under Creative Commons Attribution CC-BY 4.0.
Open access licensed under Creative Commons Attribution CC-BY 4.0.
This time Burke’s team recruited 28 international race walking athletes to repeat the same approach as that used in Supernova 1. But, there was an important addition of a 20 km race at the National Road Championships approx 2-weeks after cessation of the study diet, during which time all athletes received a high-carbohydrate diet and tapered their training to “peak” for the race. The 20 km race results were compared between groups and against race times for the 10 km race by converting race times to their equivalent World Athletics points score.
The outcome?
In June 2020, the data were published and showed that athletes who chose the 3-week low-carb ketogenic habituation became less economical and performed slower in their 10 km race walk, while athletes on the 3-week high-carbohydrate diet improved their economy and their 10 km performance. Furthermore, the National Championship 20km race results showed that high-carb athletes had no further performance gain at the 20 km race compared to the prior 10 km race while the impaired 10 km performance in the low-carb keto group was not restored following 2-weeks of high-carb availability and training tapering.
Once again, the data were so clear and so closely resembled the original Supernova findings, that Burke and her colleagues poetically named the replication study: “Crisis of confidence averted: Impairment of exercise economy and performance in elite race walkers by ketogenic low carbohydrate, high fat (LCHF) diet is reproducible”.
But Supernova did not stop there… The study series is ongoing and its latest update (Supernova 4) is that keto-adaptation can occur rapidly in elite athletes — needing only 5-days of low-carbohydrate/high-fat diet to cause nutritional ketosis and elevate resting and during-exercise fat oxidation rates. But, just 5-days of low-carb ketogenic feeding reduced economy and 10km race performance, impairments that were not remedied with a single day of high-carb availability nor following a further 5-days of high-carb dietary intake despite returning muscle glycogen to baseline levels.
So, the Supernova data teach us that when a highly-trained athlete adapts to a low-carb diet they become less economical and they lose their speed during high-intensity racing. Furthermore, restoration of carbohydrate availability, even for as long as 2-weeks, does not restore impaired performance.
Does the “debate” stop there?
Nope. Now I am seeing folks proposing things like,
If a low-carb diet increases the oxygen cost of exercise, how can you remedy that?
A somewhat sensible question, given the indisputable evidence that low-carb diet habituation renders an athlete less economical. But the view that taking an economy-boosting dietary supplement like a ketone ester or beetroot to restore lost economy caused by a low-carb diet is, quite frankly, vastly over-complicating the issue.
Supplements might be the answer.
Or, rather than spending more money and “playing with fire”, a better solution to that problem is to simply choose not to jeopardise your running economy if you are looking to improve your high-intensity performance.
Furthermore, don’t distract yourself from the big things that will definitely improve your performance — phenomenal training plan design, careful training load monitoring, sleeping lots, and knowing when to rest. Also, take confidence in your new knowledge that
Experimental evidence proves the theory of bioenergetics: carbs are more economical than fats
If you’ve followed this series, you will know that theoretical bioenergetics demonstrate that
Glucose uses fewer litres of oxygen per gram than fatty acids.
Glucose produces more energy (ATP) per litre of oxygen. And,
Glucose produces ATP at a faster rate than fatty acids.
In other words, pound-for-pound, fatty acids are the less economical fuel and are less able to keep up with ATP demands when you are moving fast. So, when operating at a high intensity — at a high rate of oxygen consumption (VO2) and therefore at a high fraction of your VO2max — it is more efficient to utilise carbohydrate. Therefore, bioenergetic logic disputes the hypothesis that a low-carbohydrate diet favours a high-intensity athlete.
Since that hypothesis has now been experimentally tested under race day conditions, we know that the logic of bioenergetics holds true. So, we know that using a low-carbohydrate diet to train with a chronically low carbohydrate availability:
Reduces your muscle glycogen levels and
Lowers your muscles’ ability to transport glucose into the mitochondria, therefore lowering your muscles' ability to use glucose to produce ATP.
The consequence of which
Compromises the use of the energy substrate (glucose) that uses less oxygen to produce ATP.
Which is not useful because that then
Impairs your ability to rapidly produce ATP to fuel racing at a high fraction of your maximal aerobic capacity, i.e., when oxygen supply becomes limited.
The bottom line is, don’t be the athlete on the start line with a high maximal fat oxidation rate and a high fat max but with a low maximal carbohydrate oxidation rate and low velocity at VO2max — train to spare glycogen AND to be able to use it when you need it... Combine the best of both worlds!
Feel free to use and share this figure, but please give credit to Thomas PJ Solomon PhD @veohtu
Sounds great. And perhaps Liam Gallagher's cryptic Champagne Supernova lyric becomes a little clearer, “Slowly walking down the hall. Faster than a cannonball.” …
I suspect his nonsense was about perspective and context, which is quite relevant because those things help us conceptualise the answer to the question that you might have been hoping I would answer:
For an endurance athlete, is a high-carb diet better than a low-carb diet?
A prudent question for an athlete to ask, an answer to which I will help you understand next time…
Until next time, stay nerdy and keep empowering yourself to be the best athlete you can be by training smart.
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Who is Thomas Solomon?
My knowledge has been honed following 20+ years of running, cycling, hiking, cross-country skiing, lifting, and climbing, 15+ years of academic research at world-leading universities and hospitals, and 10+ years advising and coaching in athletic performance and lifestyle change.
I have a BSc in Biochemistry, a PhD in Exercise Science, and over 90 peer-reviewed publications in medical journals.
I'm also an ACSM-certified Exercise Physiologist (ACSM-EP), an ACSM-certified Personal Trainer (ACSM-CPT), a VDOT-certified Distance Running Coach, and a UKVRN Registered Nutritionist (RNutr).
Since 2002, I’ve conducted biomedical research in exercise and nutrition and have taught and led university courses in exercise physiology, nutrition, biochemistry, and molecular medicine.
And, with my personal experience of competing on the track (800m to 10,000m), the road (5 k to marathon), on the trails, and in the mountains, by foot, bicycle, cross-country ski, and during obstacle course races (OCR), I deeply understand what it's like to train and compete — I've been there, done it, and gotten sweat, mud, and tears on my t-shirt.
Disclaimer: I occasionally mention brands and products, but it is important to know that I don't sell recovery products, supplements, or ad space, and I'm not affiliated with / sponsored by / an ambassador for / receiving advertisement royalties from any brands. I have conducted biomedical research for which I’ve received research money from publicly-funded national research councils and medical charities, and also from private companies, including Novo Nordisk Foundation, AstraZeneca, Amylin, the A.P. Møller Foundation, and the Augustinus Foundation. I’ve also consulted for Boost Treadmills and Gu Energy on R&D grant applications, and I provide research and scientific writing services for Examine.com. Some of my articles contain links to information provided by Examine.com, but I do not receive any royalties or bonuses from those links. Importantly, none of the companies described above have had any control over the research design, data analysis, or publication outcomes of my work. I research and write my content using state-of-the-art, consensus, peer-reviewed, and published scientific evidence, combined with my empirical evidence observed in practice and feedback from athletes. My advice is, and always will be, based on my own views and opinions shaped by the scientific evidence available. The information I provide is not medical advice. Before making any changes to your habits of daily living based on any information I provide, always ensure it is safe for you to do so and consult your doctor if you are unsure.
